Influence of the organization of water-in-ionic liquid microemulsions on the size of silver particles during photoreduction

•Ag particles were synthesized by the photoreduction in water-in-ILs microemulsions.•The hydrophobicity of anions in ILs improved the stability of Ag particles.•The solubilized water molecules in the palisade layer stabilized large aggregates of Ag particles.•The growth of Ag particles was suppressed under high pressure of CO2. Metal particles of silver (Ag) have been synthesized by the photoreduction of silver nitrate (AgNO3) in water-in-ionic liquid (IL) microemulsions consisting of nonionic surfactant Tween 20 or Triton X-100, water and ionic liquid, 1-octyl-3-methylimidazolium hexafluorophosphate ([OMIm][PF6]). The formation of microemulsions as well as Ag particles produced by the photoreduction has been investigated by UV–vis, cryo-TEM, small-angle X-ray scattering (SAXS), and extended X-ray absorption fine structure (EXAFS) measurements. At the early stage of Ag particle formation under ambient pressure, the size of Ag particles in water/[OMIm][PF6]/TX-100 microemulsions was slightly larger than that in water/[OMIm][PF6]/Tween20 microemulsions. With an increase in photoirradiation time beyond 30min, precipitation of larger Ag aggregates occurred. In contrast to the preparation under ambient pressure, the growth of Ag particles and aggregates was suppressed in preparing under high pressure (25MPa) of CO2, leading to no precipitation of Ag aggregates. The average diameters of the finally-obtained metallic Ag particles prepared under high pressure of CO2 in water/[OMIm][PF6]/Tween20 and water/[OMIm][PF6]/TX-100 microemulsions were estimated from cryo-TEM to be 3.7nm and 2.8nm, respectively. By using Guinier plots at q (